1. Field of Invention
This invention relates to a low torque magnetic pick-off particularly useful in an angular momentum mass flowmeter. More specifically, it relates to an angular momentum mass flowmeter utilizing magnetic pick-offs that use specially oriented magnets on both the rotating and stationary portion of the flowmeter to generate accurately timed electrical pulses that are indicative of the angular position of the rotating elements.
2. Description of Related Art
One well known form of angular momentum mass flowmeters utilize magnets positioned on the turbine and/or the swirl generating impeller elements of the flowmeter. Reference is made to U.S. Pat. No. 3,043,141, for an illustration of such a flowmeter arrangement. In flowmeters of this type, mass flow is measured by measuring the time interval necessary for the rotating elements (the turbine and impeller to move through a relative displacement (phase) angle A. The displacement angle A is a function of the fluid torque exerted on the turbine element by the angular momentum of the fluid being measured. Sensor are mounted on the housing of the flowmeter to produce an output voltage whenever the magnets on the rotating elements pass near the sensors.
One disadvantage of the magnet and winding pick-offs flowmeters as shown in U.S. Pat. No. 3,043,141 is that they perform poorly at low flow ranges. This is because generally the attraction forces between the magnet and ferromagnetic components of the sensors (pick off coils) preclude the rotating assembly from rotating at low flow conditions. This can prevent start-up at low flows with high viscosity fluids (i.e., cold fuel). In effect, the start-up torque required to overcome the magnetic forces involved in generating the electrical signals and overcoming magnetic eddy and leakage losses reduce the accuracy and measuring range of the flowmeter.
Attempts have been made to overcome this problem. For example, FIG. 1 illustrates a prior art pick-off for a flowmeter including a field guide 2 encircling coil 3 mounted within housing 4. Magnets 5 and 6 are mounted on rotating member 7. The magnets 5 and 6 are mounted so that their respective south and north poles pass under the gap in field guide 2. The alternating magnetic field thus created is coupled by field guide 2 through coil 3. Coil 3 in turn creates an output voltage every time the pair of magnets 5 and 6 alters the magnetic field passing through it via guide 2. A major limitation of this configuration is the tendency for the magnets 5 and 6 to exert attraction forces to field guide 2 thereby impeding the motion of rotating member 7. This impediment to rotating motion of rotating member negatively influences the measurement accuracy of the flowmeter as well as its ability to perform measurements at low flow rates.
Applicant has found that by changing the configuration and orientation of the magnets and pick-off coils as compared to the prior art, the magnetic attraction forces responsible for limiting flowmeter performance can be reduced significantly. This finding extends the measuring range at low flow rates of the flowmeter as well as increasing its accuracy.
It is therefore a principal objective of the invention to provide a flowmeter having a low torque requirement imposed by the pick-off coils and associated magnetic elements over the operating range of the meter.
It is another objective of the invention to provide a low torque pick off for measuring angular displacement between moving elements internal to a flowmeter having minimal start-up torque.
It is yet another objective of the invention to provide a means for measuring the rotating velocity of internal rotating elements requiring minimal amounts of energy thereby reducing the interference with the rotating elements.
Further objectives and advantages of the invention will become readily apparent as the description thereof proceeds.